[clang] [clang][Diagnostics] Highlight code snippets (PR #66514)

[Timm Baeder via cfe-commits]

tbaederr wrote:

Does it look that bad in your local terminal as well?

It's okay-ish with the default colors here:
![Screenshot from 2024-01-30 09-48-38](https://github.com/llvm/llvm-project/assets/49720664/ca6fb7d1-d729-4fea-b0fd-0385b0e5fba2)

We might have to file a bug against compiler-explorer.

https://github.com/llvm/llvm-project/pull/66514